The invention relates to a secondary piston compressor including a crankcase, which is provided with a venting device, the venting device having an oil filter, which is connected on the inlet side to the crankcase and on the outlet side to the atmosphere. Such a secondary compressor is used for increasing the pressure of air or other environmentally compatible gases.
As a result of leakage losses, a pressure builds up in the crankcase, which makes it necessary to vent the crankcase. With normal compressors, whose suction side is at atmospheric pressure, the vented gas-oil mist can be fed to the suction side of the compressor. This is not possible with secondary compressors since they operate with an inlet pressure. The vented gas-oil mist must thus be fed through the oil filter in this case and then discharged into the atmosphere. It has transpired that the oil filters, which are wire mesh filters, are only of limited efficiency. The consequence is that oil gets into the atmosphere. This represents a burden on the environment and results also in excessive oil consumption.
It is therefore the object of the invention to prevent the undesired discharge of oil when venting the crankcase or at least drastically to reduce it.
In order to solve this object the secondary compressor of the type referred to above is characterised in accordance with the invention that the oil filter is constructed in the form of a superfine filter and has an oil collecting space on the inlet side with an oil outlet, the oil outlet communicating with the crankcase.
The invention is based on the recognition that only very little space is available for the wire mesh filter in the low turbulence, that is to say upper, region of the crankcase. The small flow area caused thereby permits only a small pressure loss if the venting is to operate effectively and reliably. The small permissible pressure loss results in turn in a correspondingly large mesh wire mesh filter. This is the explanation for the high permeability to relatively small and even relatively large oil droplets.
The superfine filter used in accordance with the invention, on the other hand, which generally comprises a microfibre fleece, can reliably retain the oil components of the mist. The residual oil content can be reduced to less than 0.01 mg/M3. However, the superfine filter requires a correspondingly larger area which is made available by the oil collecting space.
If the superfine filter is integrated into the crankcase, the wall of the oil collecting space constitutes an effective shield against the direct impingement of oil on the superfine filter. The area of the superfine filter can thus also extend into regions of the crankcase which can no longer be referred to as of low turbulence. The oil dripping down from the superfine filter can discharge continuously or intermittently out of the oil collecting space into the crankcase via its oil outlet. If a permanently opened passage is provided, this is kept so small that direct impingement of oil on the superfine filter does not occur.
In an important embodiment of the invention, it is proposed that the superfine filter be arranged in a separate housing connected to the crankcase by means of conduits, the conduit connecting the oil collecting space to the crankcase including a valve, which adopts its closed position, when the secondary compressor is operating, and its open position, when the secondary compressor is not operating.
The superfine filter is thus separated from the crankcase in this construction and is not subject to any sort of restriction as regards the fineness of its pores and the size of the filter surface. These parameters may be adjusted to an optimum separation of oil droplets and vapours whilst taking account of the permissible pressure loss. When the secondary compressor is running, the oil outlet of the oil collecting space is closed so that the oil collecting space communicates with the crankcase only via its inlet conduit, that is to say the actual venting conduit, and pressure also acts on it only via this conduit. The emptying of the oil collecting space is effected when the secondary compressor is not operating.
The valve is preferably arranged in an inclined section of the associated conduit and constructed in the form of a non-return valve, whereby it is maintained in its closed position by the pressure in the crankcase. The superfine filter thus operates automatically. The non-return valve is a very easily movable valve, whose valve member is lifted and moved into the closed position by the gentle flow during start-up of the secondary compressor. Opening occurs under the action of the weight of the valve body and the weight of the oil column above it. It is generally advantageous to associate a light valve spring with the non-return valve which promotes the movement of the valve body into the closed position.
It is of importance that the conduit leading from the oil outlet of the superfine filter to the crankcase has an inclined section for the non-return valve. Particularly advantageous conditions are produced if the superfine filter is disposed directly above the crankcase.
In order to maintain the oil loading of the superfine filter as low as possible, it is proposed that the conduit connecting the housing of the superfine filter on the inlet side to the crankcase is connected to a low turbulence region of the crankcase, preferably to an oil filler pipe, which ensures optimal shielding.
A safety valve, which is connected to the conduit connecting the housing of the superfine filter on the inlet side to the crankcase, reliably prevents the build-up of an overpressure in the crankcase, if trouble should occur in the region of the superfine filter.
In order to optimise the oil separation, at least one further filter, preferably an active carbon filter, can be connected downstream of the superfine filter on the outlet side. The residual oil content may be reduced thereby to less than 0.005 mg/m3.
The invention will be explained in more detail below with reference to a preferred exemplary embodiment in conjunction with the accompanying drawings.